On April 4, 1844, Thomas and Caleb Pratt, a father and son engineering team, were granted US Patent #3523 for their Pratt through-truss bridge design (Historic Bridges Encyclopedia). A simplified explanation of the design focuses on the arrangement of vertical and diagonal components. �The internal diagonals are under tension during balanced loading and vertical elements under compression� (Wikipedia), which provides both stability and durability; this design can be used for spans of up to 250 ft. The Historical Documents section includes a copy of the patent issued to Thomas Pratt and Caleb Pratt for this bridge design.
Importance of the Pratt Through-Truss Bridge Design
Early in our bridge history research, we contacted Julie Bowers of Workin� Bridges because of Bowers�
previous work on the restoration of the Piano bridge located in Fayette County. Ms. Bowers explained the historical importance of the Pratt through-truss bridges. �These old Pratt bridges are considered to be the first of the modern bridges and were workhorses in the development of the areas where they were built�these old bridges made it possible for people to easily get from one location to another; whenever and wherever that happens, it revolutionizes the area�s development� (Bowers 2015). Bowers also elaborated on the Pratt through-truss design: �The design made these bridges very durable and very stable; they could easily be shipped by rail from the manufacturer and assembled onsite.� She also provided information to link the design of the Pratt through-truss bridge to other bridges that became available later: many of the other bridges were based on the Pratt design. Bowers also stated, �These bridges used to be everywhere, but they�re disappearing quickly. And they need to be saved.�
The explanations provided by Ms. Bowers were confirmed experientially in several ways. First, that the Lower Elgin Road bridge was able to withstand daily use for more than 100 years provides a testament to its durability and its stability. Second, the old iron bridges that were so commonplace until fairly recently are disappearing. Finally, despite their differences, a review of several of the �newer� truss bridge designs show similarities to the Pratt design.